You may want to look in to installing an "extend a stay" fitting into your propane line which will allow you to do exactly what you're proposing. This device actually works in two ways. You can run an external stove, hot water device, whatever your hearts desire from the westy tank. Alternatively, if your onboard tank runs dry, you can hook up a portable tank to the fitting and work in in reverse, thus "extending" your stay. This is the way the device was designed and intended.  I tend to keep my Westy tank for a reserve, and use a small 5 pound or so bottles, which is easy to transport.

I don't use my extend a stay to feed my stove too often, as it kind of limits where I can set up, but it's definitely possible. Check out bus depot for the add on I'm talking about. The small 5 pound bottles are easy to transport, and that's what I've been using lately. And,  while I'm saying 5 pound, it may actually be an 8 pound. I don't have it in front of me at the moment, but it's a baby compared to the 20 pound ones.

If you're paranoid about transporting five lbs. inside, you can cut a vent hole underneath the sink cabinet, and store it there. Personally I think the larger tanks are much better built than the Coleman disposables, and people carry two and three of those things all the time without a care in the world. Far more dangerous IMO.

Troy